Plunge Type = Ramp
If the Plunge Type is set Ramp, this field specifies the distance the tool will move down the Z axis in one ramp move.
If the Plunge Type is set Helical, this field specifies the distance the tool will move down the Z axis in one circular revolution of the helix.
Plunge Type = Peck
If the Plunge Type is set to Peck, this setting changes to Retract. This is the vertical distance to pull out after one peck in order to pull out the chip. See Constant / Cutter Radius / Step Size.